Porous electrode for proton exchange membrane

ABSTRACT

A process for manufacturing a catalytic electrode includes depositing an electrocatalytic ink on a carrier, wherein the electrocatalytic ink includes an electrocatalytic material and a product polymerizable into a protonically conductive polymer. The process also includes solidifying the electrocatalytic ink so as to form an electrode wherein the composition of the product polymerizable into a protonically conductive polymer and its proportion in the ink is defined so that the electrode formed has a breaking strength greater than 1 MPa. The process further includes separating the electrode formed from the carrier.

RELATED APPLICATIONS

This application is a U.S. National Stage of international application number PCT/EP2013/050127 filed Jan. 4, 2013, which claims the benefit of the priority date of French Patent Application FR 1250163, filed on Jan. 6, 2012, the contents of which are herein incorporated by reference.

FIELD OF INVENTION

The invention relates to the production of gas by electrolysis, and in particular to devices for producing hydrogen using a proton exchange membrane in order to achieve low-temperature electrolysis of water.

BACKGROUND

Fuel cells are envisioned as systems for supplying electrical power to mass-produced automotive vehicles in the future, and for many other applications. A fuel cell is an electrochemical device that converts chemical energy directly into electrical energy. Dihydrogen is used as fuel in fuel cells. The dihydrogen is oxidized on an electrode of the cell and dioxygen from the air is reduced on another electrode of the cell. The chemical reaction produces water. The great advantage of fuel cells is that they do not emit atmospheric pollutants at the point where the electricity is generated.

One of the major difficulties facing the development of such fuel cells lies in the synthesis and supply of dihydrogen. On earth, hydrogen exists in large amounts only combined with oxygen (in the form of water), with sulfur (hydrogen sulfide), with nitrogen (ammonia) or with carbon (fossil fuels such as natural gas or crude oils). Therefore, the production of dihydrogen either requires fossil fuels to be consumed, or substantial amounts of cheap energy to be available in order to obtain it via thermal or electrochemical decomposition of water.

The most common process for producing hydrogen from water thus consists in applying the principle of electrolysis. To carry out such processes, electrolyzers equipped with a proton exchange membrane (PEM) are known. In such electrolyzers, an anode and a cathode are fixed on either side of the proton exchange membrane (to form a membrane/electrodes assembly) and brought into contact with water. A potential difference is applied between the anode and the cathode. Thus, oxygen is produced at the anode by oxidation of the water. The oxidation at the anode also generates H⁺ ions that pass through the proton exchange membrane to the cathode, electrons being supplied to the cathode by the electrical power supply. At the cathode, the H⁺ ions are reduced to generate dihydrogen.

In practice, such an electrolyzer generally comprises supply plates placed on either side of the membrane/electrodes assembly. Current collectors are placed between the supply plates and the membrane/electrodes assembly.

Such an electrolysis device is subject to undesirable effects. An issue for such a proton exchange membrane electrolyzer is to increase its efficiency and its lifetime, to decrease its manufacturing cost and to guarantee a high level of safety. These parameters are highly dependent on the manufacturing process of the electrodes.

In a first type of process for manufacturing an electrode, catalytic ink is deposited in a layer on the current collectors. On the anode side, high overvoltages are required for electrooxidation of water. Thus, the anodic potential of the electrolyzer is in general very high (>1.6 VSHE). The use of carbon-containing materials and especially of diffusing layers made of carbon (felt, paper, carbon fabrics) is thus impossible (corrosion into CO₂), whereas such materials are widely used on the cathode side. The current collectors on the anode side generally take the form of sintered parts made of porous titanium or of titanium meshes.

Depositing the electrocatalytic ink on such collectors has the major drawback of decreasing their porosity to water, thus limiting the transport of water to the catalyst. In addition, some of the catalyst material does not participate in the catalytic reaction (and therefore the amount thereof is unnecessarily large) because it is located in pores in the current collector. Also, the electrode/membrane interface is relatively poor because the effective surface area of the electrode is small. Thus, the performance of the electrolyzer is somewhat limited. Furthermore, locating the catalyst material in the pores of the current collector makes recycling the catalyst difficult. This is because it is particularly difficult to separate noble metals from the electrodes and current collectors. Moreover, such current collectors generally require precise and expensive machining operations to produce.

To solve certain of these drawbacks, a second type of manufacturing process involves depositing the electrocatalytic ink directly onto the proton exchange membrane, so as to form a layer forming an electrode.

In such a process, the membrane absorbs moisture during deposition of the ink and thus swells and deforms. Next, contraction during drying also causes deformation. These deformations are not negligible and generate mechanical stresses in deposits, which may lead to cracks on the formed electroactive layer. Such cracks decrease the electronic percolation of the electrode and thus decrease its electrical conductivity. In addition, the cracks may decrease cohesion between the electrode and the membrane. Moreover, in operation, the membrane is completely submerged in water, maximizing its degree of swelling. The mechanical stresses at the interface between the electrode and the membrane are thus maximized, inducing additional deterioration of the electrode. This deterioration of the electrode decreases the energy efficiency of the electrolyzer and its lifetime.

Another problem with forming an electrode by catalytic deposition on the membrane is the damage caused to this membrane by solvents present in the ink (ethanol or isopropanol for example). On the one hand, the solvents increase the permeability of the membrane to gases. Some of the gases produced at the anode and at the cathode thus diffuse through the proton exchange membrane. Not only does this cause problems with the purity of the gases produced, but it also causes safety problems. Specifically, the proportion of hydrogen in the oxygen must absolutely not exceed 4%, such a proportion being the lower explosive limit of hydrogen in oxygen. On the other hand, damage of the membrane by the solvents decreases its lifetime.

SUMMARY OF INVENTION

The invention aims to solve one or more of these drawbacks. Thus, the invention relates to a process for manufacturing a catalytic electrode such as defined in claim 1. The invention also relates to an electrochemical cell such as defined in claim 12. Other features and advantages of the invention will become more clearly apparent from the description given thereof below by way of completely nonlimiting example and with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of an example electrolysis device according to the invention; and

FIGS. 2 to 5 illustrate various steps of an example process for manufacturing an electrode for such an electrolysis device.

DETAILED DESCRIPTION

FIG. 1 is a schematic sectional view of an example electrolysis device 1 comprising two electrodes according to the invention. The electrolysis device 1 illustrated is equipped with a proton exchange membrane 30. An anode 32 and a cathode 31 are fixed on either side of the proton exchange membrane 30. The electrolysis device 1 furthermore comprises porous current collectors 35 and 36 making contact with the cathode 31 and the anode 32, respectively. The electrolysis device 1 also comprises electrical supply plates 33 and 34 making contact with the current collectors 35 and 36, respectively. The current collector 35 is thus interposed between the supply plate 33 and the cathode 31. The current collector 36 is interposed between the supply plate 34 and the anode 32. The electrolysis device 1 is in this case sealed by insulating clamping plates associated with a suitable seal 37.

The electrical supply plate 33 contains a water supply duct (not shown) in communication with the cathode 31 via the porous current collector 35. The electrical supply plate 33 also contains a duct (not illustrated) for removing dihydrogen, in communication with the cathode 31 via the porous current collector 35.

The electrical supply plate 34 contains a water supply duct (not shown) in communication with the anode 32 via the porous current collector 36. The electrical supply plate 34 also contains a duct (not illustrated) for removing dioxygen, in communication with the anode 32 via the porous current collector 36.

The proton exchange membrane 30 serves to allow protons to be transferred between the anode 32 and the cathode 31 and serves to separate the gases generated. The proton exchange membrane 30 also acts as an electronic insulator between the anode 32 and the cathode 31. The proton exchange membrane 30 may for example be made of a perfluorosulfonic acid polymer membrane. Such materials are especially distributed under the trade name Nafion by DuPont. Such a membrane frequently is about 100 μm in thickness.

The anode 32 and the cathode 31 are brought into contact with water. A potential difference is applied between the anode 32 and the cathode 31, by way of the electrical source 2. The voltage source 2 is configured to apply a DC voltage generally comprised between 1.3 V and 3.0 V, with a current density in the supply plates 33 and 34 generally comprised between 10 and 40,000 A/m².

When such a voltage is applied, the following reaction is obtained at the anode 32:

2H₂O→4H⁺ +4e ⁻+O₂.

An oxidation reaction of the water at the anode 32 produces dioxygen. The oxidation at the anode 32 also generates H⁺ ions that pass through the proton exchange membrane 30 to the cathode 31 and electrons are sent back to the cathode 31 by the source 2. At the cathode 31, the H⁺ ions are reduced to generate dihydrogen.

The reaction at the cathode 31 is thus the following:

2H⁺+2e⁻→H₂.

According to the invention, the cathode 31 and/or the anode 32 are formed of a layer that forms neither a coating secured to the membrane 30, nor a coating secured to a current collector 35 or 36. The cathode 31 and the anode 32 are thus each formed of a mechanically self-supporting layer.

The porosity of the collectors 35 and 36 to water is thus preserved. Use of the catalyst material of the electrodes 31 and 32 is optimized, thereby improving the performance of the electrolysis device 1. Recycling of the electrolysis device 1 is made easier due to the absence of catalyst in the current collectors 35 and 36.

The invention makes it possible to avoid any interaction between the membrane 30 and the electrodes 31, 32 during the process used to manufacture the latter. Thus, the deformation inherent to wetting and drying of the membrane 30 when an ink is applied thereto is avoided. Cracks of the electrodes 31 and 32 is thus prevented, and likewise the decreased performance that results therefrom. Degradation of the membrane 30 due to solvents possibly used when forming the electrodes 31 and 32 is thus also avoided. The impermeability of the membrane 30 to gases is thus improved.

FIGS. 2 to 4 more precisely illustrate various steps of an example process for manufacturing an electrode 31 or 32 according to the invention.

In a first step, illustrated in FIG. 2, liquid electrocatalytic ink 6 is deposited on a carrier 4. The ink 6 may be deposited by any appropriate means, for example using a coating technique, or by spraying by means of a nozzle 5 as illustrated. To produce an electrode with a uniform thickness, the ink 6 is advantageously deposited on a carrier 4 that is substantially flat and held horizontal.

As detailed below, the electrocatalytic ink 6 and the carrier 4 are advantageously produced in such a way as to adhere weakly, this being contrary to the conventional aim of making the ink adhere to a membrane or a current collector. On the one hand the electrocatalytic ink 6 comprises a product polymerizable into a protonically conductive polymer, and on the other hand an electrocatalytic material. The polymerizable product is intended to be solidified giving the electrode a certain mechanical strength while also making it possible for water and ions to diffuse to the electrocatalytic material when the electrode is assembled in the catalysis device 1. The polymerizable product may take the form of a dissolved polymer or of an ionomer.

The electrocatalytic material has catalytic properties matched to the catalytic reaction to be carried out. The electrocatalytic material may take the form of particles or nanoparticles containing metal atoms. The catalyst material may especially comprise metal oxides. In the formulations mentioned below, the electrocatalytic material is iridium oxide. Metals such as platinum, gold, silver, cobalt and ruthenium may also be used. The electrocatalytic ink 6 advantageously contains a thickening product, methylcellulose for example. The various components of the electrocatalytic ink 6 are advantageously dissolved in a solvent such as water.

To limit the adhesion between the ink 6 and the carrier 4, the latter advantageously has a roughness lower than 5 μm, and preferably lower than 3 μm. Advantageously, the carrier 4 has an interface energy lower than 60 millinewton/m, and preferably lower than 45 millinewton/m, in order to limit adhesion of the solidified ink 6. Advantageously, the carrier 4 has an interface energy higher than 20 millinewton/m, and preferably at least equal to 25 millinewton/m, in order to limit the formation of scattered drops before the ink 6 has solidified.

To limit the adhesion between the ink 6 and the carrier 4, the deposited ink has a surface tension higher than the interface energy of the carrier 4.

In a second step, illustrated in FIG. 3, the electrocatalytic ink 6 deposited on the carrier 4 is solidified. The polymerizable product then reacts to form a polymer. A film forming the electrode is thus formed on the carrier 4. The polymerizable product is for example an ionomer.

The step of solidifying the electrocatalytic ink may involve a drying intended to evaporate the solvent. Any drying process known per se may be used, especially drying by means of an oven or a flow of hot air. The electrocatalytic ink may for example be dried by placing it in an environment at a temperature comprised between 50° and 150° C.

In a third step, the electrode formed by the solidified ink 6 is separated from the carrier 4. This separation may for example be achieved by peeling or lifting the solidified ink 6 from the carrier 4 using a small force because the solidified ink 6 is not strongly bonded to the carrier 4. A weak adhesion guarantees that the electrode formed may be separated without any risk of deterioration.

In order to allow the electrode to be handled during its separation from the carrier 4, and to give it mechanical strength during its subsequent use, the composition of the polymerizable product and its proportion in the ink are defined so that the electrode formed has a breaking strength greater than 1 MPa, preferably greater than 5 MPa, and typically comprised between 1 and 15 MPa.

An electrode thus formed will advantageously have a thickness comprised between 2 and 20 μm and preferably between 5 and 10 μm.

In a fourth step, illustrated in FIG. 5, an electrode thus formed is integrated into an electrochemical cell, here an electrolysis device. Here, the electrode formed is a cathode 31 that is inserted between a proton exchange membrane 30 and a conductive current collector 35.

Advantageously, any fastening of the middle part of the electrode is avoided in order not to degrade the performance or lifetime thereof. Advantageously, at least 95% of the area of the electrode is not mechanically bonded to the current collector and not mechanically bonded to the membrane. Advantageously, the electrode is held in position by compressing it between the proton exchange membrane 30 and a current collector. A pressure of between 0.05 and 1.5 MPa will for example be applied to the electrode in order to ensure it is held in position.

Advantageously, the electrocatalytic material includes a thickening product. This thickening product advantageously comprises methylcellulose.

Trials have demonstrated that a proportion of thickening product, and in particular methylcellulose, of between 2 and 10.5% of the solid content of the ink proves to be particularly advantageous to facilitate separation of the solidified ink and the carrier 4. Optimally, this proportion of thickening product is between 3 and 6% and preferably between 4 and 5%.

To ensure the electrode preserves optimal water diffusion properties, the proportion of the polymerizable product in the ink is advantageously defined so that the electrode formed has a porosity of between 20 and 40%. The ink 6 will possibly and advantageously have a proportion of polymerizable product of between 20 and 30% of its solid content.

To facilitate the catalytic effect of the electrode formed, the proportion of electrocatalytic material is advantageously between 60 and 75% of the solid content of the ink.

The following is a first example of a possible formulation for the ink 6:

Products % by weight % by weight (solid content) IrO₂ 12.3 69.9 Nafion ionomer DE2020 4.9 27.84 Methylcellulose 0.4 2.26 Water 82.3

The following is a second example of a possible formulation for the ink 6:

Products % by weight % by weight (solid content) IrO₂ 12.3 68.33 Nafion ionomer DE2020 4.9 27.22 Methylcellulose 0.8 4.44 Water 82.0

The following is a third example of a possible formulation for the ink 6:

Products % by weight % by weight (solid content) IrO₂ 12.2 64.21 Nafion ionomer DE2020 4.8 25.26 Methylcellulose 2.0 10.53 Water 81.0

In the examples given, the various components of the ink are dissolved in water. However, it may also be envisioned to dissolve these components in alcohol or in a mixture of alcohol and water.

In the process for manufacturing the electrode described above, the electrode does not contain reinforcing fibers. Such an absence of reinforcing fibers allows the protonic permeability of the electrode to be improved and the density of incorporated electrocatalytic material to be maximized. However, it may of course be envisioned to include reinforcing fibers in the ink in order to increase its breaking strength properties.

Although the example described illustrates the integration of an electrode into an electrolysis device, the invention of course also applies to the integration of such an electrode into a fuel cell. Such an electrode may then be bonded to a membrane adapted to a fuel cell, for example having a thickness of about 25 μm. 

1. A process for manufacturing a catalytic electrode comprising the steps of: depositing an electrocatalytic ink on a carrier, said electrocatalytic ink including an electrocatalytic material and a product polymerizable into a protonically conductive polymer; solidifying the electrocatalytic ink so as to form an electrode, the composition of the product polymerizable into a protonically conductive polymer and its proportion in the ink being defined so that the electrode formed has a breaking strength greater than 1 MPa; and separating the electrode formed from the carrier.
 2. The process for manufacturing a catalytic electrode as claimed in claim 1, in which the carrier on which said electrocatalytic ink is deposited has a roughness lower than 5 μm.
 3. The process for manufacturing a catalytic electrode as claimed in claim 1, in which the carrier on which said electrocatalytic ink is deposited has an interface energy of between 20 and 60 mN/m.
 4. The process for manufacturing a catalytic electrode as claimed in claim 1, in which the electrocatalytic ink deposited has a surface tension higher than an interface energy of the carrier.
 5. The process for manufacturing a catalytic electrode as claimed in claim 1, in which the proportion of the product polymerizable into a protonically conductive polymer in the ink is defined so that the electrode formed has a porosity comprised between 20 and 40%.
 6. The process for manufacturing a catalytic electrode as claimed in claim 1, in which the electrocatalytic ink deposited comprises methylcellulose in a proportion by weight of between 2 and 10.5% of the solid content of the ink.
 7. The process for manufacturing a catalytic electrode as claimed in claim 1, in which the electrocatalytic ink deposited comprises electrocatalytic material in a proportion by weight of between 60 and 75% of a solid content of the ink.
 8. The process for manufacturing a catalytic electrode as claimed in claim 1, in which the electrocatalytic ink deposited comprises product polymerizable into a protonically conductive polymer in a proportion by weight of between 20 and 30% of the solid content of the ink.
 9. The manufacturing process as claimed in claim 1, in which said separating step is carried out by peeling the electrode formed from the carrier.
 10. A process for manufacturing an electrochemical cell comprising: manufacturing a catalytic electrode; and inserting the catalytic electrode between a proton exchange membrane and a conductive current collector; wherein manufacturing the catalytic electrode comprises the following steps: depositing an electrocatalytic ink on a carrier, said electrocatalytic ink including an electrocatalytic material and a product polymerizable into a protonically conductive polymer; solidifying the electrocatalytic ink so as to form an electrode, the composition of the product polymerizable into a protonically conductive polymer and its proportion in the ink being defined so that the electrode formed has a breaking strength greater than 1 MPa; and separating the electrode formed from the carrier.
 11. The manufacturing process as claimed in claim 11, comprising a step of holding the catalytic electrode in position by compressing the catalytic electrode between the proton exchange membrane and the current collector.
 12. An electrochemical cell, comprising: a proton exchange membrane; a conductive current collector; and an electrode inserted between the proton exchange membrane and the current collector, the electrode containing a protonically conductive polymer and an electrocatalytic material, the electrode having a breaking strength greater than 1 MPa and at least 95% of its area not being mechanically bonded to the conductive current collector and not being mechanically bonded to the proton exchange membrane.
 13. Electrochemical cell as claimed in claim 12, in which the electrode contains methylcellulose in a proportion by weight of between 2 and 10.5%.
 14. The process for manufacturing a catalytic electrode as claimed in claim 1, in which the electrocatalytic ink deposited comprises methylcellulose in a proportion by weight of between 3 and 6% of the solid content of the ink. 